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          LV06-02-网络基础-08-UDP协议
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              <table>    <tr><td align="center">参考方向  </td><td align="center">参考原文</td></tr>    <tr><td align="center" width=150px>TCP协议</td><td align="left"><a href="https://xiaolincoding.com/network/" target="_blank">图解网络介绍 | 小林coding (xiaolincoding.com)  <i class="fa fa-external-link-alt"></i> </a></td></tr>    <tr><td align="center" width=150px>UDP协议</td><td align="left"><a href="http://c.biancheng.net/view/6439.html" target="_blank">UDP协议简介 (biancheng.net) <i class="fa fa-external-link-alt"></i> </a></td></tr>    <tr><td align="center" width=150px>UDP协议</td><td align="left"><a href="https://baike.baidu.com/item/UDP/571511" target="_blank">UDP_百度百科 (baidu.com) <i class="fa fa-external-link-alt"></i> </a></td></tr></table>
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<h1 id="一、UDP协议简介"><a href="#一、UDP协议简介" class="headerlink" title="一、UDP协议简介"></a><font size=3>一、<code>UDP</code>协议简介</font></h1><h2 id="1-UDP是什么？"><a href="#1-UDP是什么？" class="headerlink" title="1. UDP是什么？"></a><font size=3>1. <code>UDP</code>是什么？</font></h2><p><code>UDP</code>，<code>User Datagram Protocol</code>即用户数据报协议，它也是一种传输层协议，它是<strong>不可靠的无连接的</strong>协议。在数据发送前，因为不需要进行连接，所以可以进行高效率的数据传输。</p>
<h2 id="2-UDP协议特点"><a href="#2-UDP协议特点" class="headerlink" title="2. UDP协议特点"></a><font size=3>2. <code>UDP</code>协议特点</font></h2><ul>
<li>没有各种连接：在传输数据前不需要建立连接，也避免了后续的断开连接。</li>
<li>不重新排序：对到达顺序混乱的数据包不进行重新排序。</li>
<li>没有确认：发送数据包无须等待对方确认。因此，使用 <code>UDP</code> 协议可以随时发送数据，但无法保证数据能否成功被目标主机接收。</li>
</ul>
<h2 id="3-适用场景"><a href="#3-适用场景" class="headerlink" title="3. 适用场景"></a><font size=3>3. 适用场景</font></h2><ul>
<li><p>发送小尺寸数据（如对<code>DNS</code>服务器进行<code>IP</code>地址查询时）</p>
</li>
<li><p>在接收到数据，给出应答较困难的网络中使用<code>UDP</code>（如：无线网络）。</p>
</li>
<li><p>适合于广播&#x2F;组播式通信中。</p>
</li>
<li><p><code>MSN/QQ/Skype</code>等即时通讯软件的点对点文本通讯以及音视频通讯通常采用<code>UDP</code>协议。</p>
</li>
<li><p>流媒体、<code>VOD</code>、<code>VoIP</code>、<code>IPTV</code>等网络多媒体服务中通常采用<code>UDP</code>方式进行实时数据传输。</p>
</li>
</ul>
<h2 id="4-与TCP的区别"><a href="#4-与TCP的区别" class="headerlink" title="4. 与TCP的区别"></a><font size=3>4. 与<code>TCP</code>的区别</font></h2><h3 id="4-1-连接方面"><a href="#4-1-连接方面" class="headerlink" title="4.1 连接方面"></a><font size=3>4.1 连接方面</font></h3><ul>
<li><code>TCP</code> 是面向连接的传输层协议，传输数据前先要建立连接。</li>
<li><code>UDP</code> 是不需要连接，即刻传输数据。</li>
</ul>
<h3 id="4-2-服务对象方面"><a href="#4-2-服务对象方面" class="headerlink" title="4.2 服务对象方面"></a><font size=3>4.2 服务对象方面</font></h3><ul>
<li><code>TCP</code> 是一对一的两点服务，即一条连接只有两个端点。</li>
<li><code>UDP</code> 支持一对一、一对多、多对多的交互通信。</li>
</ul>
<h3 id="4-3-可靠性方面"><a href="#4-3-可靠性方面" class="headerlink" title="4.3 可靠性方面"></a><font size=3>4.3 可靠性方面</font></h3><ul>
<li><code>TCP</code> 是可靠交付数据的，数据可以无差错、不丢失、不重复、按需到达。</li>
<li><code>UDP</code> 是尽最大努力交付，不保证可靠交付数据。</li>
</ul>
<h3 id="4-4-拥塞控制和流量控制"><a href="#4-4-拥塞控制和流量控制" class="headerlink" title="4.4 拥塞控制和流量控制"></a><font size=3>4.4 拥塞控制和流量控制</font></h3><ul>
<li><code>TCP</code> 有拥塞控制和流量控制机制，保证数据传输的安全性。</li>
<li><code>UDP</code> 则没有，即使网络非常拥堵了，也不会影响 UDP 的发送速率。</li>
</ul>
<h3 id="4-5-首部开销"><a href="#4-5-首部开销" class="headerlink" title="4.5 首部开销"></a><font size=3>4.5 首部开销</font></h3><ul>
<li><code>TCP</code> 首部长度较长，会有一定的开销，首部在没有使用选项字段时是 <code>20</code> 个字节，如果使用了选项字段则会变长的。</li>
<li><code>UDP</code> 首部只有 <code>8</code> 个字节，并且是固定不变的，开销较小。</li>
</ul>
<h3 id="4-6-传输方式"><a href="#4-6-传输方式" class="headerlink" title="4.6 传输方式"></a><font size=3>4.6 传输方式</font></h3><ul>
<li><code>TCP</code> 是流式传输，没有边界，但保证顺序和可靠。</li>
<li><code>UDP</code> 是一个包一个包的发送，是有边界的，但可能会丢包和乱序。</li>
</ul>
<h3 id="4-7-分片不同"><a href="#4-7-分片不同" class="headerlink" title="4.7 分片不同"></a><font size=3>4.7 分片不同</font></h3><ul>
<li><code>TCP</code> 的数据大小如果大于 <code>MSS</code> 大小，则会在传输层进行分片，目标主机收到后，也同样在传输层组装 <code>TCP</code> 数据包，如果中途丢失了一个分片，只需要传输丢失的这个分片。</li>
<li><code>UDP</code> 的数据大小如果大于<code>MTU</code>大小，则会在 <code>IP</code> 层进行分片，目标主机收到后，在 <code>IP</code> 层组装完数据，接着再传给传输层。</li>
</ul>
<h1 id="二、UDP报文格式"><a href="#二、UDP报文格式" class="headerlink" title="二、UDP报文格式"></a><font size=3>二、<code>UDP</code>报文格式</font></h1><h2 id="1-报文图示"><a href="#1-报文图示" class="headerlink" title="1. 报文图示"></a><font size=3>1. 报文图示</font></h2><p><code>UDP</code>报文格式如下图所示：</p>
<img data-src="https://fanhua-picture.oss-cn-hangzhou.aliyuncs.com/01%E5%B5%8C%E5%85%A5%E5%BC%8F%E5%BC%80%E5%8F%91/01HQ%E8%AF%BE%E7%A8%8B%E4%BD%93%E7%B3%BB/LV06-%E7%BD%91%E7%BB%9C%E7%BC%96%E7%A8%8B/LV06-02-%E7%BD%91%E7%BB%9C%E5%9F%BA%E7%A1%80-08-UDP%E5%8D%8F%E8%AE%AE/img/image-20220623202813106.png" alt="image-20220623202813106" style="zoom:50%;" />

<h2 id="2-字段说明"><a href="#2-字段说明" class="headerlink" title="2. 字段说明"></a><font size=3>2. 字段说明</font></h2><table>
    <tr><td align="center" width=150px>源端口  </td><td align="left">这个字段占据 UDP 报文头的前 16 位，通常包含发送数据报的应用程序所使用的 UDP 端口。接收端的应用程序利用这个字段的值作为发送响应的目的地址。这个字段是可选的，所以发送端的应用程序不一定会把自己的端口号写入该字段中。如果不写入端口号，则把这个字段设置为 0。这样，接收端的应用程序就不能发送响应了。</td></tr>
    <tr><td align="center" width=150px>目的端口</td><td align="left">接收端计算机上 UDP 软件使用的端口，占据 16 位。</td></tr>
    <tr><td align="center" width=150px>包长度  </td><td align="left">该字段占据 16 位，表示 UDP 数据报长度，包含 UDP 报文头和 UDP 数据长度。因为 UDP 报文头长度是 8 个字节，所以这个值最小为 8。</td></tr>
    <tr><td align="center" width=150px>校验和  </td><td align="left">该字段占据 16 位，可以检验数据在传输过程中是否被损坏。</td></tr>
</table>

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